Doesn't sound good MOB, I will try and make the right sort of noises until Gizze somes along
I presume you ran new cables from each sub back the the amp seperately, normally I would have said that you have one of them out of phase so that they are cancelling each other out, but as you say the subs don't move that much I am not so sure.
Amp settings for maximum bass to check if it is working properly should be:
Low pass switch ON
Low pass frequency set to 250Hz, fully clockwise (gives 0-250Hz to the subs)
With the sub level set to 8V it gives you the minimum gain but you shouldn't need to take that down too far as the head unit output is around that level.
I presume that you have the base turned up on the head unit?
Did you get the 2 or 4 ohm subs?